Heat the olive oil in a large skillet over medium heat. Add the onions and garlic. Sauté until softened, about 8 minutes.
Add the chipotle chiles, adobo sauce, dried oregano and red pepper flakes. Sauté for 2 minutes. Remove from heat.
In a 6-qt slow cooker, add the Roma tomatoes and whole peeled tomatoes. Top with the onion mixture and stir to combine.
Cook on HIGH for 4 - 6 hours.
Using a handheld blender, puree until smooth. You can also transfer the soup to a blender or food processor to puree.
Return to the slow cooker. Stir in the sea salt and cracked pepper. Reduce temperature to LOW to keep warm until ready to serve.
This soup can be served two ways: TexMex or Italian. Pick your favorite and garnish as recommended. Serve piping hot!
This is my slightly adapted version of the Slow-Cooker Chipotle-Tomato Soup from Southern Living Magazine.